The present invention is an anhydrous process for the rearrangement of chlorine end-terminated polyorganosiloxanes to form organosiloxy end-blocked polyorganosiloxanes. The process is especially useful for the rearrangement of chlorine end-terminated polyorganosiloxanes having one or more hydrogen atoms bonded to silicon. The process comprises contacting a mixture comprising a chlorine end-terminated polyorganosiloxane and an organodisiloxane with a rearrangement catalyst. The rearrangement catalyst is effective in facilitating a chlorine and organosiloxy exchange between the chlorine end-terminated polyorganosiloxane and the organodisiloxane and may further effect rearrangement of the silicon to oxygen bonds within the chlorine end-terminated polyorganosiloxane and the silicon to oxygen bonds within the organosiloxy end-blocked polyorganosiloxanes. In the present process, the organodisiloxane serves as a source of (1) an organosiloxy end-blocker which displaces the chlorines of the chlorine end-terminated polyorganosiloxanes and (2) an organosilyl component which binds with the displaced chlorine to form a volatile organochlorosilane. The volatile organochlorosilane is continuously removed from the process.
Since the early work of Hyde, U.S. Pat. No. 2,457,677, issued Dec. 28, 1948, it has been known that diorganodihalosilanes in the presence of water hydrolyze to form a mixture of cyclic diorganosiloxanes and short-chained linear diorganosiloxanes along with hydrogen chloride as a by-product. Hyde also observed in this patent that the presence of a triorganosilane in the process could result in polydiorganosiloxanes that were triorganosiloxy end-blocked.
Hyde, U.S. Pat. No. 2,467,976, issued Apr. 19, 1949, teaches that the viscosity of polydimethylsiloxanes produced by the hydrolysis of diorganodichlorosilanes could be increased by refluxing the polydimethylsiloxanes with hydrochloric acid effecting rearrangement of silicon to oxygen bonds within the polydimethylsiloxanes. In U.S. Pat. No. 2,779,776, Hyde further teaches that the acid concentration is important in determining the equilibrium viscosity of the polydimethylsiloxane products.
Wilcock, U.S. Pat. No. 2,491,843, issued Dec. 20, 1949, described processes, similar to those described by Hyde, for the production of polyorganohydrosiloxanes.
The hydrolysis of diorganodichlorosilanes to form a mixture of cyclic siloxanes and short-chain linear siloxanes with the liberation of chlorine continues to be an important first step in commercial processes for producing higher molecular weight polyorganosiloxanes. However, the chlorine displaced from the diorganodichlorosilanes during the hydrolysis process creates several problems. For example, it is known that residual chloride present in polyorganosiloxane fluids can reduce shelf-life of the fluids by effecting viscosity changes. Therefore, any process for producing higher molecular weight polyorganosiloxanes from a hydrolyzate must be capable of controlling the level of chlorine present in the polyorganosiloxanes. A second problem is disposition of the displaced chlorine after recovery from the process. Because of the economic value of chlorine and the cost of disposal of chlorine, it is preferred to recover the chlorine for use in the same or different processes. Several processes have been described to deal with these problems but each suffers from various shortcomings.
For example, in one process, the hydrolysis is run in the presence of a stoichiometric excess of water, resulting in the production of cyclosiloxanes, short-chained hydroxyl terminated polysiloxanes, and aqueous hydrogen chloride. The partitioning of the chlorine into the aqueous phase is an exothermic process requiring that the reactor be cooled to maintain a desired temperature. Furthermore recovery of the chlorine from the process in the form of anhydrous hydrogen chloride must typically be accomplished by an energy intensive distillation of a HCl-H.sub.2 O azeotrope.
In another process, as exemplified by Hajjar, U.S. Pat. No. 4,609,751, issued Sep. 2, 1968, the process is run with about a stoichiometric equivalence of water with the consequential generation of anhydrous hydrogen chloride. The resultant product is cyclosiloxanes and chlorine end-terminated polyorganosiloxanes. Although this process provides a satisfactory solution to the recovery of displaced chlorine, to assure satisfactory low-levels of chlorine in the final polyorganosiloxane product a multi-step process is typically required. The chlorine-terminated polyorganosiloxanes are typically washed with one or more portions of water to form hydroxyl-terminated polyorganosiloxanes and a weak aqueous hydrogen chloride solution which can be returned to the process. These hydroxyl-terminated polyorganosiloxanes can then be further processed to produce long-chain polyorganosiloxanes. This additional wash step increases the cost of producing long-chain polyorganosiloxanes.
An additional problem associated with the described processes is that the process conditions can be sufficiently severe to cause reaction of desired silicon-bonded hydrogen when present in the polyorganosiloxanes. Therefore, these process are not entirely suitable for the production of polyorganohydrosiloxanes.
Therefore, an objective of the present invention is to provide a process for converting chlorine-end terminated polyorganosiloxanes to polyorganosiloxanes without first converting the end-terminal chlorine to end-terminal hydroxyl. A second objective is to provide a process for making polyorganosiloxanes end-blocked with organosiloxy groups. A third objective is to provide a process where the polyorganosiloxanes have an acceptably low level of residual chloride. A fourth objective is to provide a process where the chlorine displaced from the chlorine-terminated polyorganosiloxane is recovered in a form suitable for use in other processes. A fifth objective of the present invention is to provide a process where chlorine end-terminated polyorganohydrosiloxanes can be processed to form polyorganohydrosiloxanes without significant reaction of the silicon-bonded hydrogen.